a. Technical Field
The present disclosure generally relates to systems and methods for measuring the unambiguous roll angle of a projectile.
b. Description of the Related Art
Various approaches exist for determining the roll angle of a projectile using polarized radiation. For example, U.S. Pat. No. 5,039,029 describes an approach where a monitor determines, from the ground, the orientation (roll and attitude) of a missile by observing the character of a signal transmitted therefrom. By transmitting a linearly-polarized signal to a ground receiver, the missile identifies a given plane fixed with respect to the body frame of the missile. The plane is identified at the ground receiver by comparing the ratio of the signal received in any two cross-polarized directions. To resolve the attendant 180° ambiguity, the monitor employs an antenna array disposed on the missile that is switchable, back-and-forth, between two equal power lobes. The monitor operates with a single transmitter, and the lobe structure of the transmitted signal is obtained using a linearly-polarized antenna array with electronic phase switching between the aforementioned equal power lobes. This approach requires that the projectile contain a transmitter, therefore creating additional costs and requiring valuable space on the projectile.
Another approach for determining the roll angle of a projectile using polarized radiation is disclosed and described in U.S. Pat. No. 5,414,430. The system described therein includes a transmitter for emitting position-determining polarized radiation (space wave) in the direction of the projectile, and a receiver arranged in the projectile for receiving the emitted radiation. The emitted space wave is composed of a carrier wave reference with frequency f1, which has been phase-modulated with a modulation frequency f2, wherein f2<f1 and wherein f2 forms a submultiple of the carrier wave frequency f1. Similar to the approach in U.S. Pat. No. 5,039,029, this approach requires that the projectile contain a transmitter.
Yet another approach for determining the roll angle of a projectile is to use polarization-sensitive reflectors. U.S. Pat. No. 5,490,643 describes a system including an optical device located in the rear of a projectile, the optical device having a retro-reflector fitted with a polarizer and, at the projectile firing station, a light source whose beam illuminates the rear of the projectile. A light flux analyzer deduces the roll angle of the projectile from the direction of polarization of the light-flux reflected by the projectile. The polarizer has a refraction index discontinuity positioned on the rear of the projectile, before the retro-reflector, with an angle of inclination between the direction normal to its plane of index variation and the longitudinal axis of the projectile and chosen to be greater than the Brewster angle. The light source is laterally offset laterally with respect to the firing axis of the projectile. These two measurements give rise to a modulation of intensity of the reflected light beam as a function of the roll angle plus or minus 2π, which gives, to two successive maximum values of the signal of the analyzer, different amplitudes that enable the signals to be differentiated, and therefore make it possible to remove the ambiguity of π resulting from the measurement of the direction of polarization. This approach requires space on the end of the projectile. For some missiles, rockets, and artillery pieces, there is no space available on the back of the projectile for the polarization-sensitive reflectors. For example, for some missiles, the end of the missile is occupied by motor components. As well, exhaust plumes may mask the end of the projectile.
U.S. Pat. No. 6,483,455 discloses a device for the unambiguous measurement of the angle of roll of a projectile. The system includes a radar equipped with means for processing and sending a signal to the casing of the projectile in at least one direction of incident polarization, and a set of parallel grooves made on the casing, the depth of which is modulated dissymmetrically with respect to the axis of symmetry of the projectile. The axis of symmetry of the projectile does not pass through the point of the antenna of the radar where the antenna beam is generated. The processing means analyze, in reception, a signal that is back-scattered by the casing of the projectile, the signal being modulated as a function of the angle of roll of the projectile. The modulation has two maximum local values corresponding to two angular roll positions of the projectile when the polarization is parallel to the grooves. The processing means removes the 180° ambiguity by comparing the levels of the local maximum values. Again, this approach requires space on the end of the projectile for the placement of the grooves. As previously noted, space is not always available at the end of a projectile.
Therefore, there is a need for improved systems and methods that address these and other shortcomings of the prior art.